The relationship between propagated contractions and pseudoaffective changes in blood pressure in response to intestinal distension

Spinal Reflex Control of Arterial Blood Pressure: The Role of TRP Channels and Their Endogenous Eicosanoid Modulators

The spinal cord is an important integrative center for blood pressure control. Spinal sensory fibers send projections to sympathetic preganglionic neurons of the thoracic spinal cord and drive sympathetically-mediated increases in blood pressure. While these reflexes responses occur in able-bodied individuals, they are exaggerated following interruption of descending control - such as occurs following spinal cord injury. Similar reflex control of blood pressure may exist in disease states, other than spinal cord injury, where there is altered input to sympathetic preganglionic neurons. This review primarily focuses on mechanisms wherein visceral afferent information traveling via spinal nerves influences sympathetic nerve activity and blood pressure. There is an abundance of evidence for the widespread presence of this spinal reflex arch originating from virtually every visceral organ and thus having a substantial role in blood pressure control. Additionally, this review highlights specific endogenous eicosanoid species, which modulate the activity of afferent fibers involved in this reflex, through their interactions with transient receptor potential (TRP) cation channels.

Feeding and colonic distension provoke altered autonomic responses in irritable bowel syndrome

OBJECTIVE

Alterations in autonomic balance, detectable by heart rate variability (HRV) analysis, have been shown to occur after a meal in patients with irritable bowel syndrome (IBS). There are few data on changes in sympathovagal responses in IBS to other forms of enteric stimulation such as colonic distension. The aim of this study was to determine the effects of colonic balloon distension on HRV in the fasting and the postprandial state in healthy subjects and in IBS patients.

MATERIAL AND METHODS

Eight IBS patients and 8 age- and gender-matched healthy subjects underwent unsedated descending colonic distension before and after a 1000 kcal liquid meal. Low- (LF) and high-frequency (HF) HRV band values obtained from 2-min ECG segments recorded before and during distension were compared between groups, and between fasting and postprandial states. A visual analogue scale was used to determine sensation during colonic distension.

RESULTS

HF values decreased significantly with feeding in IBS patients (p=0.01), but not in healthy subjects. The low-to-high frequency (LF/HF) ratio was significantly higher postprandially in IBS patients (p=0.02) and, additionally, was decreased (p<0.01) with colonic distension in the fed state, independently of colonic sensitivity or distending volume. Moreover, changes in the LF/HF ratio with distension in the fed versus the fasting state were negatively correlated in IBS patients but positively correlated in healthy subjects (both p<0.05).

CONCLUSIONS

IBS patients demonstrated altered autonomic responses to feeding and colonic distension. Further studies should determine whether these alterations could explain the postprandial exacerbation of symptoms in IBS.

Effects of intraduodenal glucose concentration on blood pressure and heart rate in healthy older subjects

The aims of this study were to determine whether the hypotensive and heart rate responses to small intestinal glucose infusion are dependent on the glucose concentration. Eight healthy subjects, aged 65-78 years, were studied on 3 separate days in random order. Each subject received intraduodenal infusions of 50 g of glucose in either 300 mL (16.7%), 600 mL (8.3%), or 1200 mL (4.1%) of saline (0.9%) at a rate of 3 kcal/min for 60 minutes (t = 0-60 minutes), followed by saline (0.9%) for a further 60 minutes (t = 60-120 minutes). During the infusions, blood pressure (systolic and diastolic) and heart rate were measured every 3 minutes, and blood glucose concentrations every 15 minutes. Systolic and diastolic blood pressure fell (P < .0001), and heart rate and blood glucose increased (P = .0001 for both) over time, during all 3 infusions. Between t = -2-120 minutes, there was no difference in systolic blood pressure (P = .20), diastolic blood pressure (P = .61), or heart rate (P = .09) over the study days. There was also no significant difference in the glycemic response to the infusions. We conclude that in healthy older subjects, glucose concentration does not affect the blood pressure or heart rate responses to intraduodenal glucose and that, therefore, the magnitude of the postprandial fall in blood pressure induced by oral glucose is likely to depend primarily on the small intestinal glucose load.

Facilitation of gastric compliance and cardiovascular reaction by repeated isobaric distension of the rat stomach

Gastric distension causes cardiovascular reactions and enhances gastric compliance. Here, we investigated how these responses are related to each other, whether they change upon repeated distension and which neural mechanisms are involved. Mean arterial blood pressure (MAP) in phenobarbital-anaesthetized rats was recorded from a carotid artery and gastric compliance determined with an electronic barostat. Runs of intermittent gastric distension were generated by stepwise increments (5 mmHg) of intragastric (IG) pressure. While gastric compliance peaked at IG pressures of 20 mmHg, the change in MAP (predominantly hypotension) was largest at IG pressures beyond 30 mmHg. Repeated distension enhanced the MAP response to IG pressures beyond 35 mmHg, whereas gastric compliance was facilitated primarily at IG pressures below 20 mmHg. This facilitation of gastric compliance depended on the magnitude of the preceding distension. The MAP response to distension was enhanced by nitric oxide synthase inhibition, inhibited by subdiaphragmatic vagotomy but hardly affected by coeliac ganglionectomy. The facilitation of gastric compliance was changed by vagotomy in a complex manner but left unaltered by the other interventions. These findings show that isobaric gastric distension elicits both MAP and gastric compliance responses whose characteristics, mechanisms and sensitization properties differ profoundly.

A quantitative approach to recording peristaltic activity from segments of rat small intestine in vivo

We have developed methods that allow correlation of propulsive reflexes of the intestine with measurements of intraluminal pressure, fluid movement and spatio-temporal maps of intestinal wall movements for the first time in vivo. A segment of jejunum was cannulated and set up in a Trendelenburg recording system while remaining connected to the vascular and nerve supply of the anaesthetized rat. The resting intraluminal pressure in intact intestine was 2-4 mmHg. Hydrostatic pressures of 2, 4, 8 and 16 mmHg were imposed. At a baseline pressure of 4 mmHg, propulsive waves generated pressures of 9 +/- 1 mmHg, that progressed oral to anal at 2-5 mm s(-1). Individual propulsive waves propelled 0.8 +/- 0.4 mL of fluid. The frequency of propulsive waves increased with pressure, but peristaltic efficiency (mL per contraction) decreased with pressure increase between 4 and 16 mmHg. Atropine, as a bolus, transiently blocked peristalsis, but caused maintained block when infused. Hexamethonium blocked propulsive contractions. Inhibition of nitrergic transmission converted regular peristalsis to non-propulsive contractions. These studies demonstrate the utility of an adapted Trendelenburg method for quantitative investigation of motility and pharmacology of enteric reflexes in vivo.

Effects of the peripherally acting NK receptor antagonist, SB-235375, on intestinal and somatic nociceptive responses and on intestinal motility in anaesthetized rats

We investigated the effects of the selective NK(3) tachykinin receptor antagonist, SB-235375, on noxious signalling from gut and skin and on intestinal motility in anaesthetized rats. We also measured penetrance into brain and spinal cord. Nociceptive responses in reaction to colorectal distension and skin pinch were assessed by recording the electromyogram (EMG) from the external oblique muscle (a visceromotor response). Motility was measured by recording intraluminal pressure waves during changes in baseline pressure in the jejunum. Colorectal compliance was assessed by measuring luminal pressure change during isovolumic distension. SB-235375 (20 mg kg(-1), by i.v. bolus) reduced the EMG response to colorectal distension by over 90%. The reduction was slow at onset, peaked at about 60 min, and lasted for over 2 h. Responses to noxious skin pinch were unchanged. Amplitudes of propulsive waves in the jejunum were slightly reduced, but their frequency of occurrence was unchanged. SB-235375 decreased colorectal compliance by 5-10%. There was undetectable penetration of i.v. SB-235375 into brain or spinal cord. We conclude that SB-235375 acts peripherally to substantially reduce nociceptive signalling from colorectum without affecting noxious signalling from skin and with little effect on intestinal motility.

Physiological and behavioural responses to duodenal pain in freely moving rats

Luminal distention of the intestine can be aversive in humans and laboratory animals, and hypersensitivity to distention is found in functional gastrointestinal disorders. Current animal models either require anaesthesia or acute balloon intubation or use implanted balloons of irritant materials, for which the aversive quality of distention and physiological responses have not been well characterised. We report here that silicone balloon catheters implanted in the duodenum via the stomach have long patency without obvious tissue damage. Balloon inflation in freely moving rats caused passive avoidance learning and classic 'pain' postures, as well as graded cardiovascular responses which can be recorded telemetrically. The method should make long-lasting studies of pharmacological and environmental effects on visceral sensitivity more feasible.

Analysis of factors that determine the compliance of rat jejunum to distension in vivo

Distension of the intestine is commonly used to elicit reflex responses at other sites in the gastrointestinal tract, and also to evaluate pain of intestinal origin. The sensory neurones, that initiate the reflexes or pain responses, react to the forces generated in the wall of the intestine. Thus, the responses of the intestine at the site of distension, particularly changes in contractile activity, influence the signals from the gut. In the present work we have analysed the relationship between distension and pressure changes in the jejunum of the rat, in vivo. Isovolumic distension for 5 min caused an initial pressure increase which declined quickly in the first 30 s, and then declined more slowly. Phasic pressure increases were superimposed on the baseline pressure change. Hexamethonium blocked the phasic pressure increases, whereas the initial rapid and subsequent slower pressure decline during distension persisted. Inhibition of nitric oxide synthase (NOS) increased intraluminal pressure and caused increased frequency and irregularity of phasic pressure increases. However, the decline in jejunal pressure during distension was not changed by inhibition of NOS. The pressure decline during isovolumic distension was similar whether saline or paraffin oil were used to distend the intestine, indicating that the decline was not due to increased hydrostatic pressure causing water and electrolyte to cross the mucosal epithelium from the lumen to the intestinal interstitium. Hyoscine had no significant effect on the pressure profile when the intestine was distended. However, when the systemic or the local circulation of the jejunum was infused with nicardipine, the pressure that was achieved during isovolumic distension was less, although the rate of change in pressure during the slow decline was similar. It is concluded that distension evokes phasic pressure increases in the jejunum, that are nerve-mediated, and increases the tension in the wall through a stretch-activated increase in contractile force generated by the circular muscle. The decline in pressure during maintained distension is primarily a consequence of visco-elastic properties of the wall of the intestine.